Alkali free glass and method for producing alkali free glass

ABSTRACT

The present invention relates to an alkali-free glass having a strain point of 725° C. or higher, an average thermal expansion coefficient at from 50 to 300° C. of from 30×10 −7  to 40×10 7 /° C., a temperature T 2  at which a glass viscosity is 10 2  dPa·s of 1,710° C. or lower, and a temperature T 4  at which a glass viscosity is 10 4  dPa·s of 1,320° C. or lower, the alkali-free glass including, in terms of mol % on the basis of oxides, SiO 2 : 66 to 70, Al 2 O 3 : 12 to 15, B 2 O 3 : 0 to 1.5, MgO: more than 9.5 and 13 or less, CaO: 4 to 9, SrO: 0.5 to 4.5, BaO: 0 to 1, and ZrO 2 : 0 to 2, in which MgO+CaO+SrO+BaO is from 17 to 21, MgO/(MgO+CaO+SrO+BaO) is 0.4 or more, MgO/(MgO+CaO) is 0.4 or more, MgO/(MgO+SrO) is 0.6 or more, and the alkali-free glass does not substantially contain an alkali metal oxide.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.13/912,642, filed Jun. 7, 2013, which is a continuation of InternationalApplication No. PCT/JP2011/77960, filed Dec. 2, 2011 which is based uponand claims the benefits of priority to Japanese Application No.2010-272674, filed Dec. 7, 2010. The entire contents of all of the aboveapplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an alkali-free glass that is suitableas various substrate glasses for a display and substrate glasses for aphotomask, does not substantially contain an alkali metal oxide and canbe subjected to float forming.

BACKGROUND OF THE INVENTION

The following characteristics have conventionally been required invarious substrate glasses for a display, particularly substrate glasseshaving a metal or an oxide thin film formed on the surface thereof.

(1) In the case where an alkali metal oxide is contained, alkali metalions diffuse in the thin film, resulting in deterioration of filmcharacteristics. For this reason, alkali metal ions are notsubstantially contained.

(2) A strain point is high such that deformation of a glass andshrinkage (thermal shrinkage) due to structure stabilization of a glasscan be minimized when exposed to high temperature in a thin filmformation step.

(3) A glass has sufficient chemical durability to various chemicals usedin semiconductor formation. Particularly, the glass has durability tobuffered hydrofluoric acid (BHF: mixed liquid of hydrofluoric acid andammonium fluoride) for etching SiO_(x) or SiN_(X), a medicinal solutioncontaining hydrochloric acid used for etching ITO, various acids (nitricacid, sulfuric acid and the like) used for etching an metal electrode,and an alkaline of a resist stripping solution.

(4) Defects (bubble, striae, inclusion, pit, flaw and the like) are notpresent in the inside and on the surface.

In addition to the above requirements, the recent years are underfollowing situations.

(5) Reduction in weight of a display is required, and a glass itself isrequired to have small density.

(6) Reduction in weight of a display is required, and decrease inthickness of a substrate glass is desired.

(7) In addition to the conventional amorphous silicon (a-Si) type liquidcrystal display, a polycrystal silicon (p-Si) type liquid crystaldisplay in which heat treatment temperature is slightly high has beganto be produced (a-Si: about 350° C.→p-Si: 350 to 550° C.).

(8) A glass having small average thermal expansion coefficient isrequired in order to improve productivity and increasing thermal shockresistance by increasing a temperature-rising rate in a heat treatmentfor liquid display preparation.

On the other hand, dry etching progresses and requirement to BHFresistance is becoming weakened. Many glasses conventionally used areglasses containing 6 to 10 mol % of B₂O₃ in order to improve BHFresistance. However, B₂O₃ has the tendency to decrease a strain point.The following glasses are exemplified as an alkali-free glass that doesnot contain B₂O₃ or contains B₂O₃ in small amount.

Patent Document 1 discloses SiO₂-Al₂O₃-Sro glass that does not containB₂O₃. However, a temperature required for melting is high, and thiscauses difficulty in production.

Patent Document 2 discloses SiO₂-Al₂O₃-Sro crystallized glass that doesnot contain B₂O₃. However, a temperature required for melting is high,and this causes difficulty in production.

Patent Document 3 discloses a glass containing B₂O₃ in an amount of from0 to 3% by weight. However, an average thermal expansion coefficient atfrom 50 to 300° C. exceeds 40×10⁻⁷/° C.

Patent Document 4 discloses a glass containing B₂O₃ in an amount of from0 to 5 mol %. However, an average thermal expansion coefficient at from50 to 300° C. exceeds 50×10⁻⁷/° C.

Patent Document 5 discloses a glass containing B₂O₃ in an amount of from0 to 5 mol %. However, thermal expansion is large and density is alsolarge.

To solve the problems in the glasses described in Patent Documents 1 to5, an alkali-free glass described in Patent Document 6 is proposed. Thealkali-free glass described in Patent Document 6 has high strain point,can be formed by a float process, and is considered to be suitable foruses such as a display substrate and a photomask substrate.

A production method of high quality p-Si TFT includes a solid phasecrystallization method. However, to carry out the method, it is requiredto further increase a strain point.

On the other hand, to comply with the demand in glass productionprocess, particularly melting and forming, a glass is required to havelower viscosity, particularly lower temperature T₄ at which a glassviscosity is 10⁴ dPa·s.

BACKGROUND ART Patent Documents

Patent Document 1: JP-A-62-113735

Patent Document 2: JP-A-62-100450

Patent Document 3: JP-A-4-325435

Patent Document 4: JP-A-5-232458

Patent Document 5: U.S. Pat. No. 5,326,730

Patent Document 6: JP-A-10-45422

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

An object of the present invention is to provide an alkali-free glassthat solves the above disadvantages, has high strain point and lowviscosity, particularly low temperature T₄ at which a glass viscosity is10⁴ dPa·s, and is easily subjected to float forming.

Means for Solving the Problems

The present invention provides an alkali-free glass having a strainpoint of 725° C. or higher, an average thermal expansion coefficient atfrom 50 to 300° C. of from 30×10⁻⁷ to 40×10⁷/° C., a temperature T₂ atwhich a glass viscosity is 10² dPa·s of 1,710° C. or lower, and atemperature T₄ at which a glass viscosity is 10⁴ dPa·s of 1,320° C. orlower, the alkali-free glass comprising, in terms of mol % on the basisof oxides:

SiO₂ 66 to 70, Al₂O₃ 12 to 15, B₂O₃ 0 to 1.5, MgO more than 9.5 and 13or less, CaO 4 to 9, SrO 0.5 to 4.5, BaO 0 to 1, and ZrO₂ 0 to 2,wherein MgO+CaO+SrO+BaO is from 17 to 21, MgO/(MgO+CaO+SrO+BaO) is 0.4or more, MgO/(MgO+CaO) is 0.4 or more, MgO/(MgO+SrO) is 0.6 or more, andthe alkali-free glass does not substantially contain an alkali metaloxide.

The present invention provides an alkali-free glass having a strainpoint of 725° C. or higher, an average thermal expansion coefficient atfrom 50 to 300° C. of from 30×10⁻⁷ to 40×10⁻⁷/° C., a temperature T₂ atwhich a glass viscosity is 10² dPa·s of 1,710° C. or lower, and atemperature T₄ at which a glass viscosity is 10⁴ dPa·s of 1,320° C. orlower, the alkali-free glass comprising, in terms of mol % on the basisof oxides:

SiO₂ 66 to 70, Al₂O₃ 12 to 15, B₂O₃ 0 to 1.5, MgO 5 to 9.5, CaO 4 to 11,SrO 0.5 to 4.5, BaO 0 to 1, and ZrO₂ 0 to 2,wherein MgO+CaO+SrO+BaO is more than 18.2 and 21 or less,MgO/(MgO+CaO+SrO+BaO) is 0.25 or more, MgO/(MgO+CaO) is 0.3 or more,MgO/(MgO+SrO) is 0.6 or more, Al₂O₃x(MgO/(MgO+CaO+SrO+BaO)) is 5.5 ormore, and the alkali-free glass does not substantially contain an alkalimetal oxide.

Advantage of the Invention

The alkali-free glass of the present invention is a glass that issuitable for a display substrate, a photomask substrate and the likethat are for uses at high strain point, and is easily subjected to floatforming.

MODE FOR CARRYING OUT THE INVENTION

Composition range of each component is descried below. In the case whereSiO₂ is less than 66% (mol %; unless otherwise indicated, thereinafterthe same), a strain point is not sufficiently increased, a thermalexpansion coefficient is increased and a density is increased. In thecase where the amount exceeds 70%, a melting performance is decreased,and a devitrification temperature is increased. The amount is preferablyfrom 67 to 70%.

Al₂O₃ inhibits phase separation property of a glass, decreases a thermalexpansion coefficient and increases a strain point. However, in the casewhere the amount is less than 12%, the effect is not achieved, and othercomponents increasing expansion are increased. As a result, a thermalexpansion becomes large. In the case where the amount exceeds 15%, amelting performance of a glass may be deteriorated, and adevitrification temperature may be increased. Therefore, the amount ispreferably 14.5% or less, more preferably 14% or less, and still morepreferably from 12.2 to 13.8%.

B₂O₃ improves a melting reaction property of a glass, and decreases adevitrification temperature. Therefore, B₂O₃ can be added up to 1.5%.However, in the case where the amount is too large, a strain point isdecreased. Therefore, the amount is preferably 1% or less. Consideringenvironmental load, it is preferred that B₂O₃ is not substantiallycontained.

MgO has the characteristics that it does not increase expansion and doesnot excessively decrease a strain point, among alkali earths. MgO alsoimproves a melting performance.

In a first embodiment of the alkali-free glass of the present invention,the MgO content is more than 9.5% and 13% or less. In the case where theamount is 9.5% or less, the effect by the addition of MgO as describedabove is not achieved. However, in the case where the amount exceeds13%, a devitrification temperature may be increased. The amount is morepreferably 12.5% or less, 12% or less, and 11.5% or less.

On the other hand, in a second embodiment of the alkali-free glass ofthe present invention, the MgO content is from 5 to 9.5%. In the casewhere the amount is less than 5%, the effect by the addition of MgO asdescribed above is not achieved. The amount is more preferably 6% ormore, and 7% or more. However, in the case where the amount exceeds9.5%, a devitrification temperature may be increased. The amount is morepreferably 9.3% or less, and 9% or less.

CaO has the characteristics that it does not increase expansion and doesnot excessively decrease a strain point, next to MgO among alkaliearths. CaO also improves a melting performance.

In the first embodiment of the alkali-free glass of the presentinvention, the CaO content is from 4 to 9%. In the case where thecontent is less than 4%, the effect by the addition of CaO as describedabove is not sufficiently achieved. However, in the case where thecontent exceeds 9%, a devitrification temperature may be increased and aphosphorus that is impurities in lime (CaCO₃) that is CaO raw materialmay be incorporated in large amount. The content is more preferably 7%or less, 6% or less and 5% or less.

On the other hand, in the second embodiment of the alkali-free glass ofthe present invention, the CaO content is from 4 to 11%. In the casewhere the content is less than 4%, the effect by the addition of CaO asdescribed above is not sufficiently achieved. The content is preferably5% or more. However, in the case where the content exceeds 11%, adevitrification temperature may be increased and a phosphorus that isimpurities in lime (CaCO₃) that is CaO raw material may be incorporatedin large amount. The content is more preferably 10% or less, 9% or less,7% or less and 6% or less.

SrO improves a melting performance without increasing a devitrificationtemperature of a glass. In the case where the content is less than 0.5%,the effect is not sufficiently achieved. The content is preferably 1.0%or more, and more preferably 2.0% or more. However, in the case wherethe content exceeds 4.5%, an expansion coefficient may be increased. Thecontent is preferably 4.0% or less, and 3.5% or less.

BaO is not essential, but can be contained to improve a meltingperformance. However, in the case where the content is too large, anexpansion and a density of a glass are excessively increased. Therefore,the content is 1% or less. The content is more preferably 0.5% or less,and it is preferred that BaO is not substantially contained.

ZrO₂ may be contained up to 2% in order to decrease a glass meltingtemperature or to accelerate crystal precipitation during burning. Inthe case where the content exceeds 2%, a glass becomes unstable, ordielectric constant c of a glass is increased. The content is preferably1.5% or less.

In the first embodiment of the alkali-free glass of the presentinvention, in the case where the total content of MgO, CaO, SrO and BaOis less than 17%, a melting performance becomes poor. In the case wherethe total content is more than 21%, there may be a difficulty that athermal expansion coefficient cannot be decreased. The total content ispreferably 18% or more, and 20% or less.

On the other hand, in the second embodiment of the alkali-free glass ofthe present invention, in the case where the total content of MgO, CaO,SrO and BaO is 18.2% or less, a melting performance becomes poor. In thecase where the total content is 21% or more, there may be a difficultythat a thermal expansion coefficient cannot be decreased. The totalcontent is preferably 20% or less.

In the first embodiment of the alkali-free glass of the presentinvention, when the total content of MgO, CaO, SrO and BaO satisfies theabove and the following three requirements are satisfied, a strain pointcan be increased without increasing a devitrification temperature, andadditionally, a viscosity of a glass, particularly a temperature T₄ atwhich the glass viscosity is 10⁴ dPa·s, can be decreased.

MgO/(MgO+CaO+SrO+BaO) is 0.4 or more, and preferably 0.45 or more.

MgO/(MgO+CaO) is 0.4 or more, preferably 0.52 or more, and morepreferably 0.55 or more.

MgO/(MgO+SrO) is 0.6 or more, and preferably 0.7 or more.

In the second embodiment of the alkali-free glass of the presentinvention, when the total content of MgO, CaO, SrO and BaO satisfies theabove and the following three requirements are satisfied, a strain pointcan be increased without increasing a devitrification temperature, andadditionally, a viscosity of a glass, particularly a temperature T₄ atwhich the glass viscosity is 10⁴ dPa·s, can be decreased.

MgO/(MgO+CaO+SrO+BaO) is 0.25 or more, preferably 0.3 or more, morepreferably 0.4 or more, and still more preferably 0.45 or more.

MgO/(MgO+CaO) is 0.3 or more, preferably 0.4 or more, more preferably0.52 or more, and still more preferably 0.55 or more.

MgO/(MgO+SrO) is 0.6 or more, and preferably 0.7 or more.

In the second embodiment of the alkali-free glass of the presentinvention, when Al₂O₃x(MgO/(MgO+CaO+SrO+BaO)) is 5.5 or more, a Young'smodulus is increased, which is preferred. Al₂O₃x(MgO/(MgO+CaO+SrO+BaO))is preferably 5.75 or more, more preferably 6.0 or more, still morepreferably 6.25 or more, and particularly preferably 6.5 or more.

Incidentally, the alkali-free glass of the present invention does notcontain an alkali metal oxide in a content exceeding an impurity level(that is, does not substantially contain) in order to avoid theoccurrence of deterioration in characteristics of a metal or a thinoxide film provided on a glass surface during panel production. For thesame reason, it is preferred that P₂O₅ is not substantially contained.Furthermore, to facilitate recycle of a glass, it is preferred that PbO,As₂O₃ and Sb₂O₃ are not substantially contained.

In addition of the above components, to improve a melting performance, arefining and a formability (float formability) of a glass, ZnO, Fe₂O₃,SO₃, F, Cl and SnO₂ can be added in the total amount of 5% or less.

The alkali-free glass of the present invention has a strain point of725° C. or higher, and preferably higher than 730° C., and therefore athermal shrinkage during panel production can be suppressed. Further,solid phase crystallization method can be applied as a production methodof p-Si TFT.

In the alkali-free glass of the present invention, the strain point ismore preferably 735° C. or higher. When the strain point is 735° C. orhigher, the glass is suitable for uses at high strain point (forexample, a display substrate or a lighting substrate for an organic EL,or a thin display substrate or a lighting substrate, having a thicknessof 100 μm or less).

Forming of a sheet glass having a thickness of 100 μm or less has thetendency that drawing rate during forming becomes fast. As a result, afictive temperature of a glass is increased, and a compaction of a glassis increased easily. In this case, when a glass is a high strain pointglass, the compaction can be inhibited.

Also, the alkali-free glass of the present invention has a glasstransition point of preferably 760° C. or higher, more preferably 770°C. or higher, and still more preferably 780° C. or higher.

Moreover, the alkali-free glass of the present invention has an averagethermal expansion coefficient at 50 to 300° C. of from 30×10⁻⁷ to40×10⁻⁷/° C., has a large thermal shock resistance and can increaseproductivity when producing a panel. It is preferred in the alkali-freeglass of the present invention that the average thermal expansioncoefficient at 50 to 300° C. is from 35×10⁻⁷ to 40×10⁻⁷/° C.

Furthermore, the alkali-free glass of the present invention has aspecific gravity of preferably 2.65 or less, more preferably 2.64 orless, and still more preferably 2.62 or less.

Moreover, according to the alkali-free glass of the present invention, atemperature T₂ at which viscosity η is 10² poise (dPa·s) is 1,710° C. orlower, preferably lower than 1,710° C., more preferably 1,700° C. orlower and still more preferably 1,690° C. or lower. Therefore, meltingof the glass is relatively easy.

Furthermore, according to the alkali-free glass of the presentinvention, a temperature T₄ at which viscosity η is 10⁴ poise is 1,320°C. or lower, preferably 1,315° C. or lower, more preferably 1,310° C. orlower, and still more preferably 1,305° C. or lower. Therefore, it issuitable for float forming

When the alkali-free glass of the present invention has adevitrification temperature of 1,350° C. or lower, forming by a floatprocess becomes easy, which is preferred. The devitrificationtemperature is preferably 1,340° C. or lower, and more preferably 1,330°C. or lower.

The devitrification temperature in the present description is an averagevalue of a maximum temperature at which crystals precipitate on thesurface of a glass and in the inside thereof, and a minimum temperatureat which crystals do not precipitate, by placing crushed glass particleson a platinum dish, conducting heat treatment for 17 hours in anelectric furnace controlled to constant temperature, and observing withan optical microscope after the heat treatment.

Also, the alkali-free glass of the present invention preferably has aYoung's modulus of 84 GPa or more, furthermore 86 GPa or more,furthermore 88 GPa or more, and furthermore 90 GPa or more.

Moreover, the alkali-free glass of the present invention preferably hasa photoelastic constant of 31 nm/MPa/cm or less.

When a glass substrate has a birefringence by stress generated in aliquid crystal display panel production step and during using a liquidcrystal display, display of black becomes gray, and the phenomenon thatcontrast of a liquid crystal display is decreased is sometimesrecognized. When the photoelastic constant is 31 nm/MPa/cm or less, thisphenomenon can be inhibited small. The photoelastic constant ispreferably 30 nm/MPa/cm or less, more preferably 29 nm/MPa/cm or less,still more preferably 28.5 nm/MPa/cm or less, and particularlypreferably 28 nm/MPa/cm or less.

Considering easiness of securing other properties, the photoelasticconstant is preferably 25 nm/MPa/cm or less.

The photoelastic constant can be measured by a disk compression method.

Also, the alkali-free glass of the present invention preferably has adielectric constant of 5.6 or more.

In the case of In-Cell type touch panel (touch sensor is incorporated ina liquid crystal display panel) as described in JP-A-2011-70092, it isbetter that the glass substrate has higher dielectric constant from thestandpoints of improvement in sensing sensitivity of a touch sensor,decrease in drive voltage and electric power saving. When the dielectricconstant is 5.6 or more, sensing sensitivity of a touch sensor isimproved. The dielectric constant is preferably 5.8 or more, morepreferably 6.0 or more, still more preferably 6.2 or more, andparticularly preferably 6.4 or more.

The dielectric constant can be measured according to the methoddescribed in JIS C-2141 (1992).

The alkali-free glass of the present invention can be produced by, forexample, the following method. Raw materials of each component generallyused are mixed so as to obtain a target component, and the resultingmixture is continuously introduced in a melting furnace, and heated atfrom 1,500 to 1,800° C. to melt the same. The molten glass obtained isformed into a given sheet thickness by a float process, followed byannealing and cutting. Thus, a sheet glass can be obtained.

The alkali-free glass of the present invention has relatively lowmelting performance. Therefore, the following materials are preferablyused as raw materials of each component.

(Silicon Source)

Silica sand can be used as a silicon source of SiO₂. When silica sand,in which a median diameter D₅₀ is from 20 to 27 μm, the proportion ofparticles having a particle size of 2 μm or less is 0.3% by volume orless and the proportion of particles having a particle size of 100 μm ormore is 2.5% by volume or less, is used, the silica sand can be meltedwhile inhibiting the aggregation of the silica sand. This facilitatesmelting of the silica sand, and an alkali-free glass having less bubblesand having high homogeneity and flatness is obtained, which ispreferred.

Incidentally, the “particle size” in the present specification means asphere equivalent size (primary particle size in the present invention)of silica sand, and is specifically a particle size in particle sizedistribution of a powder measured by a laser diffraction/scatteringmethod.

Moreover, the “median diameter D₅₀” in the present specification means aparticle size where, in particle size distribution of a powder measuredby a laser diffraction method, volume frequency of particles having aparticle size larger than a certain particle size occupies 50% of thatof the whole powder. In other words, the “median diameter D₅₀” means aparticle size when the cumulative frequency is 50% in particle sizedistribution of a powder measured by a laser diffraction method.

The “proportion of particles having a particle size of 2 μm or less” andthe “proportion of particles having a particle size of 100 μm or more”in the present specification are measured by particle size distributionwith, for example, laser diffraction/scattering method.

It is preferred that the median diameter D₅₀ of silica sand is 25 μM orless because melting of the silica sand becomes easier.

Moreover, it is particularly preferred that the proportion of particleshaving a particle size of 100 μm or more in silica sand is 0% becausemelting of the silica sand becomes easier.

(Alkaline Earth Metal Source)

An alkali earth metal compound can be used as the alkaline earth metalsource.

Specific examples of the alkaline earth metal compound includecarbonates such as MgCO₃, CaCO₃, BaCO₃, SrCO₃ and (Mg, Ca)CO₃(dolomite); oxides such as MgO, CaO, BaO and SrO; and hydroxides such asMg(OH)₂, Ca(OH)₂, Ba(OH)₂ and Sr(OH)₂. When a hydroxide of an alkalineearth metal is contained in a part or the whole of the alkaline earthmetal source, the amount of unmelted SiO₂ component during melting glassraw materials is decreased, which is preferred. In the case where theamount of unmelted SiO₂ component contained in silica sand is increased,the unmelted SiO₂ is incorporated in bubbles when the bubbles aregenerated in a glass melt, and accumulates near the surface layer of theglass melt. This causes difference in compositional ratio of SiO₂between the surface layer of the glass melt and part other than thesurface layer. As a result, homogeneity of a glass is deteriorated, andadditionally, flatness is decreased.

The content of the hydroxide in the alkaline earth metal is preferablyfrom 15 to 100 mol % (in terms of MO), more preferably from 30 to 100mol % (in terms of MO), and still more preferably from 60 to 100 mol %(in terms of MO), of 100 mol % of the alkaline earth metal source (interms of MO, provided that M is an alkaline earth metal element). Whenthe content falls within the range, the amount of the unmelted SiO₂during melting glass raw materials is decreased, which is morepreferred.

The amount of the unmelted SiO₂ during melting glass raw materials isdecreased with increasing molar ratio of the hydroxide in the alkalineearth metal source. Therefore, higher molar ratio of the hydroxide ispreferred.

Specifically, a mixture of a hydroxide of an alkaline earth metal and acarbonate, a hydroxide alone of an alkaline earth metal, and the likecan be used as the alkaline earth metal source. At least one of MgCO₃,CaCO₃, and (Mg, Ca)(CO₃)₂ (dolomite) is preferably used as thecarbonate. At least one of Mg(OH)₂ and Ca(OH)₂ is preferably used as thehydroxide of the alkaline earth metal. Mg(OH)₂ is particularlypreferably used.

(Boron Source)

When the alkali-free glass contains B₂O₃, a boron compound can be usedas the boron source of B₂O₃. Specific examples of the boron compoundinclude orthoboric acid (H₃BO₃), metaboric acid (HBO₂), tetraboric acid(H₂B₄O₇) and boric anhydride (B₂O₃). In the production of generalalkali-free glass, orthoboric acid is used from the standpoints ofinexpensive cost and easy availability.

In the present invention, the boron source preferably contains boricanhydride in a content of from 10 to 100 mass % (in terms of B₂O₃) of100 mass % (in terms of B₂O₃) of the boron source. When the boricanhydride contained is 10 mass % or more, aggregation of glass materialsis inhibited, and improvement effect in reduction of bubbles,homogeneity and flatness is obtained. The content of the boric anhydrideis more preferably from 20 to 100 mass %, and still more preferably from40 to 100 mass %.

As a boron compound other than boric anhydride, orthobaric acid ispreferred from the standpoints of inexpensive cost and easyavailability.

EXAMPLES

In the following examples, Examples 1 to 11 and 14 to 31 are WorkingExamples, and Examples 12 and 13 are Comparative Examples. Raw materialsof each component were mixed so as to obtain a target composition, andmelted at a temperature of from 1,500 to 1,600° C. using a platinumcrucible. In melting, stirring was conducted using a platinum stirrer tohomogenize a glass. The molten glass was flown out, and formed in aplate shape, followed by annealing.

Tables 1 to 5 show glass composition (unit: mol %), thermal expansioncoefficient at 50 to 300° C. (unit: ×10⁻⁷/° C.), strain point (unit: °C.), glass transition point (unit: ° C.), specific gravity, Young'smodulus (GPa) (measured by ultrasonic method), temperature T₂(temperature at which a glass viscosity η is 10² poise, unit: ° C.)giving an indication of melting performance and temperature T₄(temperature at which a glass viscosity η is 10⁴ poise, unit: ° C.)giving an indication of float formability and fusion formability, ashigh temperature viscosity values, devitrification temperature (unit: °C.), photoelastic constant (unit: nm/MPa/cm) (measured by diskcompression method) and dielectric constant (measured by the methoddescribed in JIS C-2141).

Incidentally, in Tables 1 to 5, values in parentheses are calculatedvalues.

TABLE 1 Example Example Example Example Example Example Example Mol % 12 3 4 5 6 7 SiO₂ 67.4 68.4 68.4 67.9 67.4 68.4 68.4 Al₂O₃ 13.5 13.5 13.513.5 13.5 13.5 13.5 B₂O₃ 0 0 0 0 0 0 0 MgO 10.7 10.7 10.0 10.5 11.7 11.09.7 CaO 5.2 5.2 7.1 7.1 4.2 6.1 5.2 SrO 3.2 2.2 1.0 1.0 3.2 1.0 4.2 BaO0 0 0 0 0 0 0 ZrO₂ 0 0 0 0 0 0 0 MgO + CaO + SrO + BaO 19.1 18.1 18.118.6 19.1 18.1 19.1 MgO/(Mg + CaO + SrO + BaO) 0.56 0.6 0.6 0.6 0.6 0.60.5 MgO/(MgO + CaO) 0.67 0.7 0.6 0.6 0.7 0.6 0.7 MgO/(MgO + SrO) 0.770.8 0.9 0.9 0.8 0.9 0.7 Al₂O₃ × 7.56 0.6 0.6 0.6 0.6 0.6 0.5 (MgO/(MgO +CaO + SrO + BaO)) Average thermal expansion 37.0 34.7 35.3 37.9 37.935.5 37.7 coefficient (×10⁻⁷/° C.) Strain point (° C.) (731) (743) (740)(741) (734) (743) (734) Glass transition point (° C.) 783 795 792 793786 795 787 Specific gravity 2.57 2.55 2.54 2.54 2.57 2.53 2.59 Young’smodulus (GPa) (90.2) (89.5) (88.1) (89.5) 89.6 (89.1) (90.1) T₂ (° C.)1648 (1677) (1686) (1673) (1645) (1682) 1652 T₄ (° C.) 1307 (1314)(1301) (1291) (1293) (1298) 1310 Devitrification temperature (° C.) 12961320 or 1320 or 1310 1320 or 1320 or 1275 higher higher higher higherPhotoelastic constant (nm/MPa/cm) (29.0) (29.4) (29.8) (29.6) (28.7)(29.6) (28.9) Dielectric constant (6.55) (6.43) (6.45) (6.51) (6.54)(6.44) (6.55)

TABLE 2 Mol % Example 8 Example 9 Example 10 Example 11 Example 12Example 13 SiO₂ 67.9 67.9 67.1 67.0 68.7 68.4 Al₂O₃ 13.5 13.5 13.5 13.514.0 13.5 B₂O₃ 0 0 0 0 0.5 0 MgO 9.0 9.0 9.8 9.7 7.1 7.6 CaO 8.6 7.1 5.35.4 6.5 7.1 SrO 1.0 2.5 4.3 4.4 3.2 3.4 BaO 0 0 0 0 0 0 ZrO₂ 0 0 0 0 0 0MgO + CaO + SrO + BaO 18.6 18.6 19.4 19.5 16.8 18.1 MgO/(Mg + CaO +SrO + BaO) 0.48 0.48 0.51 0.50 0.42 0.42 MgO/(MgO + CaO) 0.51 0.56 0.650.64 0.52 0.52 MgO/(MgO + SrO) 0.90 0.78 0.78 0.69 0.69 0.69 Al₂O₃ ×6.48 6.48 6.89 6.75 5.88 5.67 (MgO/(MgO + CaO + SrO + BaO)) Averagethermal expansion 36.3 38.0 38.2 39.7 36.4 38.1 coefficient (×10⁻⁷/° C.)Strain point (° C.) (740) (740) (735) (733) 737 744 Glass transitionpoint (° C.) 791 792 785 783 796 796 Specific gravity 2.55 2.57 2.602.60 2.56 2.57 Young’s modulus (GPa) 87.9 (89.3) 89.3 (90.5) 90.8 90.9T₂ (° C.) 1653 1656 1647 1644 1705 1698 T₄ (° C.) 1309 1310 1303 12991327 1321 Devitrification temperature (° C.) 1295 1285 1285 1290 12851295 Photoelastic constant (nm/MPa/cm) (30.0) (29.6) (28.8) (28.8)(30.0) (29.8) Dielectric constant (6.53) (6.51) (6.59) (6.60) (6.34)(6.46)

TABLE 3 Example Example Example Example Example Example Example ExampleMol % 14 15 16 17 18 19 20 SiO₂ 66.5 66.5 66.2 66.2 67.5 67.5 67.5 Al₂O₃13 13.2 13.3 13.7 13.5 13.8 13.3 B₂O₃ 0 0 0 0 0.5 1 0 MgO 12.7 12.2 10.511.3 10.7 9.8 11.1 CaO 4.7 5.5 8.5 4.9 4.5 4.5 5.2 SrO 3.1 2.6 1.5 3.93.3 3.4 2.3 BaO 0 0 0 0 0 0 0.6 ZrO₂ 0 0 0 0 0 0 0 MgO + CaO + SrO + BaO20.5 20.3 20.5 20.1 18.5 17.7 19.2 MgO/(Mg + CaO + SrO + BaO) 0.62 0.600.51 0.56 0.58 0.55 0.58 MgO/(MgO + CaO) 0.73 0.69 0.55 0.70 0.70 0.690.68 MgO/(MgO + SrO) 0.80 0.82 0.88 0.74 0.76 0.74 0.83 Al₂O₃ × 8.057.93 6.81 7.70 7.81 7.64 7.69 (MgO/(MgO + CaO + SrO + BaO)) Averagethermal expansion 37.7 37.3 39.1 38.7 36.2 35.5 (35.9) coefficient(×10⁻⁷/° C.) Strain point (° C.) (726) (729) (732) (730) (731) (730)(741) Glass transition point (° C.) 794 791 795 794 796 793 Specificgravity 2.58 2.58 2.57 2.60 2.57 2.56 (2.56) Young’s modulus (GPa) 91.391.4 91.4 90.7 89.8 89.1 (88.8) T₂ (° C.) 1649 1649 (1629) 1653 (1647)1677 (1681) T₄ (° C.) 1294 1296 (1273) 1300 (1310) 1315 (1312)Devitrification temperature (° C.) 1312 1312 1312 1312 Photoelasticconstant (nm/MPa/cm) 29.9 29.6 30.0 28.9 29.2 28.0 (29.4) Dielectricconstant (6.66) (6.66) (6.73) (6.67) (6.46) (6.39) (6.62)

TABLE 4 Example Example Example Example Example Example Example Mol % 2122 23 24 25 26 27 SiO₂ 67.5 67.5 67.5 66.1 66.1 67.6 67 Al₂O₃ 12.7 13.313.7 14.1 13.8 13 13.7 B₂O₃ 0 0 0.5 1 0 0 0.8 MgO 11.3 9.3 8.8 8.8 8.58.2 5 CaO 5.4 6.5 7.3 5.5 8.3 8.8 11 SrO 2.5 3.4 2.2 4.5 3.3 2.4 2.5 BaO0 0 0 0 0 0 0 ZrO₂ 0.6 0 0 0 0 0 0 MgO + CaO + SrO + BaO 19.2 19.2 18.318.8 20.1 19.4 18.5 MgO/(Mg + CaO + SrO + BaO) 0.59 0.48 0.48 0.47 0.420.42 0.27 MgO/(MgO + CaO) 0.68 0.59 0.55 0.62 0.51 0.48 0.31 MgO/(MgO +SrO) 0.82 0.73 0.80 0.66 0.72 0.77 0.67 Al₂O₃ × 7.47 6.44 6.59 6.60 5.845.49 3.70 (MgO/(MgO + CaO + SrO + BaO)) Average thermal expansion (35.2)38.4 37.0 38.4 39.6 39.9 39.3 coefficient (×10⁻⁷/° C.) Strain point (°C.) (745) (734) (736) (727) (734) (736) (736) Glass transition point (°C.) 795 798 792 796 797 800 Specific gravity (2.55) 2.58 2.55 2.60 2.602.57 2.57 Young’s modulus (GPa) (88.8) 89.9 89.9 89.2 90.6 86.7 84.7 T₂(° C.) (1674) (1646) (1662)) 1653 (1622) 1670 (1664) T₄ (° C.) (1310)(1314) (1312) 1299 (1297) 1309 (1319) Devitrification temperature (° C.)1287 1287 Photoelastic constant (nm/MPa/cm) 27.9 28.4 29.1 28.0 29.027.6 Dielectric constant (6.60) (6.56) (6.49) (6.55) (6.72) (6.58)(6.55)

TABLE 5 Example Example Example Example Mol % 28 29 30 31 SiO₂ 66.5 66.8  67.5  67.5  Al₂O₃ 13.5  13.6  13.3  12.7  B₂O₃ 0.5  1.2  0   0  MgO 9   6.2  8.2  8.4  CaO 4.5  9.3  7.9  8.1  SrO 6   2.9  2.5  2.7 BaO 0   0   0.6  0   ZrO₂ 0   0   0   0.6  MgO + CaO + 19.5  18.4  19.2 19.2  SrO + BaO MgO/(Mg + CaO + 0.46 0.34 0.43 0.44 SrO + BaO)MgO/(MgO + CaO) 0.67 0.40 0.51 0.51 MgO/(MgO + SrO) 0.60 0.68 0.77 0.76Al₂O₃ × (MgO/(MgO + 6.23 4.58 5.68 5.56 CaO + SrO + BaO)) Averagethermal expansion 39.0  39.2  (37.9)  (37.2)  coefficient (×10⁻⁷/° C.)Strain point (° C.) (726)    (731)    (740)    (744)    Glass transition790    792    point (° C.) Specific gravity 2.56 2.62 (2.58) (2.57)Young's modulus (GPa) 84.7  85.4  (89)    (89)    T₂ (° C.) (1612)    1661     (1672)     (1665)     T₄ (° C.) (1317)     1302     (1314)    (1312)     Devitrification 1312     temperature (° C.) Photoelasticconstant 29.7  29.7  (29.3)  (29.6)  (nm/MPa/cm) Dielectric constant(6.58) (6.50) (6.56) (6.54)

As is apparent from the tables, the glasses of the examples are all thatthe thermal expansion coefficient is low as 30×10⁻⁷ to 40×10⁻⁷1° C., andthe strain point is high as 725° C. or higher. It is seen that theglasses can sufficiently withstand heat treatment at high temperature.

The temperature T₂ giving an indication of melting performance isrelatively low as 1,710° C. or lower, and melting is easy. Thetemperature T₄ giving an indication of formability is 1,320° C. orlower, and forming by a float process is easy. Furthermore, thedevitrification temperature is 1,350° C. or lower. Therefore, it isconsidered that the glasses are free from the trouble thatdevitrification occurs during float forming.

The photoelastic constant is 31 nm/MPa/cm or less, and when the glassesare used as a glass substrate for a liquid crystal display, decrease incontrast can be inhibited.

Furthermore, the dielectric constant is 5.6 or more, and when theglasses are used as a glass substrate for In-Cell type touch panel,sensing sensitivity of a touch sensor is improved.

While the present invention has been described in detail with referenceto specific embodiments thereof, it will be apparent to one skilled inthe art that various changes and modifications can be made thereinwithout departing from the spirit and scope thereof.

Incidentally, the present application is based on Japanese PatentApplication No. 2010-272674 filed on Dec. 7, 2010, and the contents areincorporated herein by reference.

INDUSTRIAL APPLICABILITY

The alkali-free glass of the present invention has high strain point,can be formed by a float process, and is suitable for uses such as adisplay substrate and a photomask substrate. Further, the alkali-freeglass is also suitable for uses such as a substrate for solar cell.

The invention claimed is:
 1. An alkali-free glass having a strain pointof 725° C. or higher, an average thermal expansion coefficient at from50 to 300° C. of from 30×10⁻⁷ to 40×10⁻⁷/° C., a temperature T₂ at whicha glass viscosity is 10² dPa·s of 1,710° C. or lower, and a temperatureT₄ at which a glass viscosity is 10⁴ dPa·s of 1,320° C. or lower, thealkali-free glass comprising, in terms of mol % on the basis of oxides:SiO₂ 66 to 70, Al₂O₃ 12 to 14.1, B₂O₃   0 to 1.5, MgO   5 to 9.5, CaO  4to 11, SrO 0.5 to 4.5, BaO 0 to 1, and ZrO₂ 0 to 2,

wherein MgO+CaO+SrO+BaO is more than 18.2 and 21 or less,MgO/(MgO+CaO+SrO+BaO) is 0.25 or more, MgO/(MgO+CaO) is 0.3 or more,MgO/(MgO+SrO) is 0.6 or more, Al₂O₃×(MgO/(MgO+CaO+SrO+BaO)) is 5.5 ormore, and the alkali-free glass does not substantially contain an alkalimetal oxide.
 2. The method for producing the alkali-free glass accordingto claim 1, wherein a silica sand, in which a median diameter D₅₀ isfrom 20 to 27 μm, a proportion of particles having a particle size of 2μm or less is 0.3 vol % or less, and a proportion of particles having aparticle size of 100 μm or more is 2.5 vol % or less, is used as asilicon source of SiO₂.
 3. The method for producing the alkali-freeglass according to claim 1, wherein an alkaline earth metal sourcecontaining a hydroxide of an alkaline earth metal in an amount of from15 to 100 mol % (in terms of MO, provided that M is an alkaline earthmetal element, and hereinafter the same) of 100 mol % of the alkalineearth metal source is used as the alkaline earth metal source of MgO,CaO, SrO and BaO.
 4. The method for producing the alkali-free glassaccording to claim 1, wherein a silica sand, in which a median diameterD₅₀ is from 20 to 27 μm, a proportion of particles having a particlesize of 2 μm or less is 0.3 vol % or less, and a proportion of particleshaving a particle size of 100 μm or more is 2.5 vol % or less, is usedas a silicon source of SiO₂; and an alkaline earth metal sourcecontaining a hydroxide of an alkaline earth metal in an amount of from15 to 100 mol % (in terms of MO, provided that M is an alkaline earthmetal element, and hereinafter the same) of 100 mol % of the alkalineearth metal source is used as the alkaline earth metal source of MgO,CaO, SrO and BaO.
 5. The alkali-free glass according to claim 1, whereinSrO is included in an amount of 2.0 to 4.5 mol % on the basis of oxides.6. The alkali-free glass according to claim 5, Al₂O₃ is included in anamount of 12.2 to 13.8 mol % on the basis of oxides.
 7. The alkali-freeglass according to claim 6, CaO is included in an amount of 4 to 6 mol %on the basis of oxides.
 8. The alkali-free glass according to claim 7,wherein the strain point is 780° C. or higher.
 9. The alkali-free glassaccording to claim 1, having a glass transition point of 760° C. orhigher, a specific gravity of 2.65 or less, a Young's modulus of 84 GPaor more, the devitrification temperature of 1,340° C. or lower, aphotoelastic constant of 31 nm/MPa/cm or less, and a dielectric constantof 5.6 or more.
 10. The alkali-free glass according to claim 1, Al₂O₃ isincluded in an amount of 12 to 14 mol % on the basis of oxides.
 11. Thealkali-free glass according to claim 1, Al₂O₃ is included in an amountof 12.2 to 13.8 mol % on the basis of oxides.
 12. The alkali-free glassaccording to claim 11, CaO is included in an amount of 4 to 6 mol % onthe basis of oxides.
 13. The alkali-free glass according to claim 12,wherein the strain point is 770° C. or higher.
 14. The alkali-free glassaccording to claim 1, CaO is included in an amount of 4 to 9 mol % onthe basis of oxides.
 15. The alkali-free glass according to claim 14,wherein the strain point is 770° C. or higher.
 16. The alkali-free glassaccording to claim 1, CaO is included in an amount of 4 to 7 mol % onthe basis of oxides.
 17. The alkali-free glass according to claim 16,wherein the strain point is 780° C. or higher.
 18. The alkali-free glassaccording to claim 1, wherein the strain point is 770° C. or higher. 19.The alkali-free glass according to claim 1, wherein the strain point is780° C. or higher.